As interest grows in reducing the use of chemical fertilizers and synthetic pesticides, producers are now paying more attention to soil quality, with some looking at soil inoculants to help improve soil health. But without a conducive soil environment, inoculants added to the soil may be a short-term fix or, worse yet, a waste of money.

Soil inoculants are groups of bacteria or fungi that are added to the soil to enhance plant nutrient uptake and the plant’s drought tolerance. Common soil inoculants include rhizobacteria and mycorrhizal fungi, which are important to agriculture, because they improve water and nutrient uptake, particularly phosphorus, suppress plant diseases, and protect soil from erosion. The fungi make external threads called hyphae, which help hold soil together.

Various soil inoculants are sold for improving plant establishment and performance and reducing post-plant stress, although many products are not independently tested.

"Soil inoculants (microorganisms like bacteria and fungi) won’t last or persist in the soil if the environment is not conducive," said Dr. Lynne Carpenter-Boggs, coordinator of Washington State University’s Biologically Intensive Agriculture and Organic Farming Program. But she noted that some soil inoculants can be effective.

Mixed results

Scientists know that mycorrhizal fungi are beneficial to the soil, but researchers are still trying to learn how they work in the field. Researchers have learned that certain cultural practices, like repeated use of soil fumigants, high amounts of fertilizer, or excessive tilling can harm mycorrhizal fungal populations.

Research literature shows mixed results regarding claims that soil inoculants increase plant survival and enhance growth.

"There are a lot of names out there for different products and inoculants, but we don’t necessarily need them," Carpenter-Boggs said.

"If you don’t have the proper soil environment, they could be a waste."

In some cases, the soil inoculants contain micronutrients, so plant benefits may be coming from the nutrients and not microorganisms.

"You may need repeated applications of the inoculants. If you don’t have the right environment, you will need to keep inoculating," she said.

Soil habitat

She believes that a faster way to build up soil organism populations is to encourage a healthy soil habitat. Each soil organism has its own special niche, she explained at the Washington State Grape Society meeting in Grandview, Washington. "Inoculation by itself is not a good way to improve soil health. You need the right soil environment, right soil neighbors, and right competition from the hosts."

Carpenter-Boggs encouraged growers to think of their soil as a "habitat," a community of microorganisms. Building up soil organic matter is a big part of developing a healthy soil, she noted, adding that food, habitat, biology, and soil chemicals are also components of healthy soils. The makeup of the soil microorganism population is dependent on soil substrate and diversity.

"Soil is the place for growing soil organisms," she summarized. "Good inoculants can be used for synergistic reasons, but they won’t make up for a poor habitat."

Key terms

Soil inoculants Groups of bacteria or fungi added to soil to enhance plant nutrient uptake and enhance the plant’s drought tolerance.

Mycorrhizal fungi Live within roots of plants in a mutually beneficial or symbiotic relationship.

Rhizobacteria Live symbiotically on nodules of the root systems, usually of legumes, and turn atmospheric nitrogen (N2) into a form that plants can use (NH3), hence the term "nitrogen-fixing bacteria."

Nodule Legume nitrogen fixation starts with the formation of a nodule, which forms along the root and is visible with the naked eye. Nodules on perennial legumes, like alfalfa and clover, will fix nitrogen through the entire growing season if conditions are favorable.

Hyphae Fine, white threads that make up the body or mycelium of a multicellular fungus.